1. Field of the Invention
The present invention relates to an apparatus and method for controlling internal reforming kinetic rates in a solid oxide fuel cell.
2. Background Art
Fuel cells are electrochemical devices that convert the chemical energy of a fuel into electricity and heat without fuel combustion. In the one type of fuel cell hydrogen gas and oxygen gas are electrochemically combined to produce electricity. The hydrogen used in this process may be obtained from natural gas or methanol while air provides the oxygen source. The only by-products of this process are water vapor and heat. Accordingly, fuel cell-powered electric vehicles reduce emissions and the demand for conventional fossil fuels by eliminating the internal combustion engine (e.g., in completely electric vehicles) or operating the engine at only its most efficient/preferred operating points (e.g., in hybrid electric vehicles). However, while fuel cell-powered vehicles have reduced harmful vehicular emissions, they present other drawbacks.
Solid oxide fuel cells (“SOFCs”) are a type of fuel cell design that is currently undergoing significant development. In some types of SOFCs, hydrocarbon fuel is fed to an anode and an oxygen containing gas is fed to the cathode. Although direct oxidation of hydrocarbon fuels at solid oxide fuel cells is desirable, typically it is necessary to reform the fuel (i.e., convert the fuel to hydrogen and carbon monoxide). Operating SOFCs by directly supplying fuel to the cell can reduce size requirements. In addition, it is possible that lower system costs and greater system efficiency can be realized by operating via direct oxidation. SOFCs may be considered to be systems or reactors that generate electrical power much like a battery, the difference being that the fuel cell is a continuous-flow reactor. The ability of SOFCs to tolerate impurities in the fuel makes it possible to process (reform) the hydrocarbon fuels within the cell. This should be contrasted to PEM fuel cells which require extremely pure hydrogen to avoid becoming poisoned, and onboard reforming does not deliver hydrogen that is sufficiently pure. Typically, nickel is used as a catalyst in SOFC anodes to raise its electrical conductivity and, fortunately, the nickel can also catalyze the reforming of hydrocarbon compounds.
There are two key factors in the thermal processes taking place in the reforming step of an operating SOFC. The reforming step is highly endothermic, that is, energy is consumed by the reaction. For example, the enthalpy for reforming reaction converting methane to carbon monoxide and hydrogen is +226 kJ/mol at 800° C. Secondly, the cell oxidation reactions are exothermic, with an enthalpy of reaction of −273 kJ/mol, assuming all of the hydrogen and carbon monoxide are consumed. Comparing the energies of these two thermal processes, there is a reasonable balance between the two reactions, averaged over an entire fuel cell surface. If the two sets of reactions happened in the same location at the same time, most of the thermal effects would cancel, and there would be only a modest temperature change in the SOFC during operation. Unfortunately. the reactions do not generally occur at the same locations. One consequence of this is that the fast reforming reactions lead to local cooling in the entrance region of SOFCs resulting in high thermal stresses. Ultimately, this localized stress can lead to cracking and failure of the SOFC cell structure.
Accordingly, there exists a need for methods of controlling internal reforming kinetic rates in a solid oxide fuel cell such that such fuel cells are subjected to lower amounts of internal stress.